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WHY does a jet engine work?
Maybe this isn't the right forum for this question but anyway:
WHY does a jet engine work?
I know how it works: compress Air -> inject fuel -> ignite -> expand -> thrust
But why does this work? Some energy from the expansion is used to turn the turbine which powers the compressor. Why is this self sustaining?
btw: It seems to be tricky to keep a jet engine running. There are examples that airliner engines sometime stall (Loss of airflow/compression in the engine)

Well it's not really self-sustaining, because energy is being added in the form of fuel.

Your provided stall example has nothing to do with airflow or compression  the engine threw a blade due to fatigue https://www.gov.uk/aaib-reports/airbus-a330-243-g-omyt-24-june-2013

What do you mean by "why"? It works because working is compatible with the laws of physics. Why are the laws of physics the way they are? That's not a question accessible to science.

Physics SE has already answered exactly this question

possible duplicate

Thank you for all your answers. I will combine your answers and the linked question from Physics SE and say that the combustion does not change the presure in the engine but the velocity. and the fan/turbine blades are designed to allow "more" air to leave the combustion chamber than enter it.

@Bigbohne, Just about everything in your last comment is wrong. Combustion does change the pressure, as well as the velocity, and how can more air leave than entered? Where does the additional air come from? More gas comes out the nozzle, but that's just because fuel is being burned and converted to CO2 and water vapor, Considering just the air that enters the front, there is actually less of it coming out the back, because some of it's oxygen content has been used in combustion.

It works because the torque produced by the turbine is equal to or greater than the rotational drag on the compressor created by the work it is doing in compressing the air flowing through it. What "why" are you in doubt about?

Some energy from the expansion is used to turn the turbine which powers the compressor

And in some engines it turns the turbine which has a large fan attached to the other end. The engine produces more power than needed to run itself.
Inside the engine, everything is a very careful balance of pressures. Basically the job done by valves in a piston engine is done by careful engineering in a turbine, which is one of the reasons they cost so much. The inflow to the combustion chamber balances the outflow to the power turbine to the point where it wants to go that way - get it a little bit out of balance and you have noisy stalls like you see in the referenced video.

The linked video shows a turbine blade failure, not a stall.

There are a few points I will make but first off a jet engine is NOT self sustaining. It takes the input of fuel to keep running. The fuel is the source of energy that keeps it running. Now lets look at the mechanics a bit, to simplify things I will use a Ramjet instead of a turbine driven jet since the compression is taken care of by the simple shape of the engine, no moving parts. Lets assume the jet is moving (ram jets dont work at zero speed).

When the engine is thrust forward air enters the front
The shape of the nozzle compresses (squeezes) the air
Fuel is mixed into the air (same as in a carb)
The air/fuel mixture enters the combustion chamber
The air/fuel mixture is ignited
The ignited mixture expands and is directed out of the back
The engine (as a result of newtons 3rd law) moves forward (opposite reaction)
7b. The air/fuel ignition contained enough energy to not only move the engine but the craft as well. Total Energy = Energy to Drive Engine + Energy to Move Object
The forward moving force causes air to enter the front of the engine
Back to step 1.

To make this all work the output energy has to be greater than the energy needed to compress the air. This energy comes from the fuel that is added to the mixture. Jet fuel has about 46 MJ/Kg.
In the case of a turbine engine we simply introduce a compressor mechanism. Instead of relying on the shape of the engine to compress the air, some of the output energy is sacrificed to spin (and operate) the compressor mechanism. It should be noted that shape does play a roll in turbine jets as well.

Great answer of how it works, but not why.

@FreeMan, and right you are.  But the OP said he knew the how, just not the why, which is why I didn't include the how.  I tend to take people at their word.  Every now and then that get me into trouble.

the how is they why, the why is nothing more than "the laws of physics" or "you are adding energy in the form of fuel". The definition of the word why is "a reason or explanation" this is an explanation. Not that I want to open a can of worms on the difference though.

I'm interested in the reason why the expansion from burning the fuel does not push against the air coming from the compressor but instead pushed towards the turbine section

Also needs a suitable source of air to certain standard.

Basically, the energy released by burning fuel is used to compress and accelerate the airflow through the engine. While you correctly note that the turbines pull some of that energy back out of the airflow and use it to keep the engine spinning, this is a relatively small fraction. Since accelerating the air backwards produces an equal magnitude, opposite direction force on the engine itself, a large forward force is applied to the engine, which is then ideally used to accelerate the airplane forwards and/or counter the aerodynamic drag forces on the airplane in order to maintain speed during flight.
As far as why the flow remains front-to-back through a turbine engine core, this has to do with which side is applying more force to the airflow. The compressor stages are designed to produce more force on the air than the turbine stages are, so the net acceleration on the air is toward the back of the engine. This question covers the topic of airflow through the engine more thoroughly. This question on Physics.SE also addresses this topic.

Perhaps OP is wondering why the hot gases flow out the back end instead of out of the front?

exactly. Why don't the gasses flow out of the front?

Newton's Third Law of motion states "For every force of action, there is an equal and opposite force of reaction."  So if the exhaust is pushed out of the back end of a jet engine with 40,000 lbs. of force, there is 40,000 lbs. of force pushing the jet, and the aircraft to which it's attached, the other direction.

Newton's Third Law answers the why question. The others answer how.

The reason it’s self sustaining is that a considerable amount of power - about 75% of the total increase in enthalpy for the air moving through the gas turbine - is used to power the compressor section of the gas core.  The remaining 25% or so of the available power is either converted into velocity by a nozzle to create reactive thrust (turbojet) or to drive a power turbine which drives a ducted fan, or a reduction gearbox which drives either a propeller or powertrain (turbofan, turboprop, turboshaft).
Now this process does require the engine core to operate at a very high speed in order to provide enough energy to sustain the compressor section of the gas core.  Typically when started, the gas core must be spun up by either an electric or pneumatic starter to around 12-20% of its maximum operating speed before fuel is introduced during the process.  Too slow, and the combustion process will not sustain and speed up the core to a typical idling speed around 60%.  Worse, this spin up will lag or delay (hang start) which can overheat and permanently damage the hot section of a turbine engine.  Pilots are always wary of this and will watch the engine temps like a hawk as fuel is introduced during the startup process.  It is the one Achilles heel of an otherwise rock solid and reliable type of aircraft powerplant.

So is it true than that the increase in energy (by burning fuel) expands in both directions (in general) but it's easier to expand out of the back?

Not exactly.  A better way to think of it is that hot gases leaving the turbine section in the jet pipe cannot backflow through the turbine due to higher pressures in this region than in the jetpipe, but can exit through the mouth of the nozzle, as atmospheric pressure here is lower than the internal pressure found in the jet pipe.

Aha ... so there is an "energy loop" here. More hot gas -> more gas leaving the engine -> higher pressure in compressor -> harder for the gas to push forward? ... That would also explain why you have to bring the engine up to speed/compression before it's self sustaining

The heat from the burning fuel means that the volume of exhaust gasses is greater than the volume of air and fuel. Since work that can be done by a pressurised fluid depends on the pressure and the volume the work that can be done by the exhaust gasses is greater than the work needed to pump the air and fuel into the engine.
After that it becomes a matter of careful engineering to make the turbine and compressor perform sufficiently across the operating range of the engine.


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How can a touchdown zone RVR report for one runway be different than the rollout report for the runway going the opposite direction?
I saw an RVR report in KSEA where 16C was 800 1200 1800 (touchdown zone, midpoint, rollout), and 34C was 1800 1200 300.  I always thought the same scatterometers used for the rollout side on runway as the touchdown zone in the opposite direction.

Transmissometers is the term usually used for the devices, rather than "scatterometers" -- don't recall coming across that one. I'd find that result highly suspect as well unless there is a (fourth) "Far End" sensor involved, but I don't think 16C/34C has those. I'd suspect either a transcription error or some difference in time between the readings. Where/when did you see this report?

I saw the RVR report on the FAA's RVR page on January 19th. https://rvr.data.faa.gov/

Don't know about this exact case, but since rvr is about contrast, could this be an issue with ambient light or sunlight making the fog brighter in one direction, hence lowering contrast? Like approaching towards sunrise is a lot worse than with the sun behind you

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What is this instrument?
I am looking to sell this instrument, but it has no part numbers or any other identifying marks other than it says "Kollsman" on the back.  Anyone know what it is and what it might be worth?

"Anyone know what it is and what it might be worth?"  -yes.

$175 https://aeroantique.com/collections/artifacts-that-have-moved-on/products/accelerometer-g-meter-an-5745?variant=38275923402

I will give you $50 for it.

Both needles are pointing to -5 in a 1G environment, which suggests to me that perhaps the instrument may be broken.  One of those needles should be pointing to "1".

https://aviation.stackexchange.com/help/someone-answers

As the face says, it's a "G meter", aka "accelerometer". It measures vertical acceleration in units of "G" (i.e. acceleration due to gravity, 9.8 m/s^2 or 32 f/s^2).
Price would vary, but here's a site with a similar instrument which sold for $75 when it was in stock: https://aeroantique.com/products/accelerometer-g-meter-1?variant=39681371274

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If commercial airliner cabin is depressurized, are pilots protected against it in any ways?
Basically, if the cabin in a large air liner is depressurized, are pilots protected in any way, specifically:

Is the cockpit separately pressurized from the rest of the cabin?
Can they keep consciousness if sudden depressurization happens?

If you search here for "oxygen", you will find many related questions with a lot of detail.

No, the cockpit is not separately pressurized. That would be impractical and not particularly useful.
However, pilots do have oxygen masks, just like the passengers. Unlike the passenger masks, these masks are generally designed to be donned extremely quickly, with one hand (they tend to have automatic straps that clamp on without needing to be manually adjusted). The masks also have a separate oxygen supply from the passenger masks, and have much more oxygen available for each mask, and have other nice features. Furthermore, when one pilot is off the flight deck, the other wears an oxygen mask that's either releasing oxygen or set to automatically do that if the plane depressurizes.
If sudden depressurization happens, there are still a few seconds of useful consciousness for most people. That time is enough for airline pilots to put on their masks. Once the crew has on oxygen masks, they can control the depressurized plane just fine (depending on how it depressurized, of course: if the tail broke off, there's not much that can really be done).

Among the "other nice features," there is a microphone in the mask so that the pilot wearing it can talk on the radio or the intercom system (to the other pilot, or to the cabin crew).

The cockpit crew have their own oxygen bottle that supplies only their masks with plenty of capacity to handle a depressurization or smoke event to landing.  The masks are quick-don and can supply 100% O2 at positive pressure.  This means you can put them on with one hand with a very quick maneuver (grab to pull out of its stowed position which inflates tubing that you put over your head which compress when you let go of the mask) and they can provide breathable air even if ambient partial pressure of oxygen would be too low to breathe.

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If left stationary at a specific height, would altimeter change the displayed value over time?
I have been reading a lot about how altimeters work, and from what I understood, most common altimeters are based of atmospheric pressure to calculate the altitude.
The question that I still have though is: If left stationary at a specific height, would altimeter change the displayed value over time? Something tell me there would be slight variation, because air pressure changes almost daily, or am I totally off with this one?

Yes, an altimeter based on atmospheric pressure will change over time. If you, say, park your airplane at an airport with an elevation of 400 feet msl and your altimeter is reading 400 feet, when you come back a day later, for example, it will read differently. If a low pressure system has come in, it will read higher. If a high pressure system has come in, it will read lower. And that is why there's a knob on the altimeter to allow you to change what it reads, and also why you should change the setting to match local pressure when you're enroute and below 18,000 feet in the U.S.

So it also takes an input of local pressure at a known height?

The variation is not necessarily slight, but can be  several hundred feet or more.

@Alexus You don't have to know the height of the local station you're passing because the setting they give is adjusted to mean sea level. For example, if the station reports 30.00 inches, that's what you would set in the Kollsman window. If the station were at 2,000 ft msl, the actual pressure there would be approximately 28 inches given the standard pressure lapse rate of approx 1 inch per 1,000 ft. However, a barometer at the station would typically read 30.00 inches because it would be adjusted to read relative to mean sea level.

@Alexus the inputs to the altimeter are actual local pressure and a reference pressure.  The difference between the two pressures determines the altitude of the local pressure level (approximately, it does not take temperature into account).  An altimeter actually tracks constant pressure levels, not true altitudes.

@casey: Though what you use to set the altimeter will vary.  If you're at a towered field or controlled airspace, you use the reference pressure.  At a small field - say in the Idaho backcountry - you set the altimeter to the field elevation.  (Though sometimes it matters which end of the field you're on :-))

@jamesqf that is still a reference pressure, though you are inferring it from field elevation (you adjust the reference pressure input until you get the output you want)

You are correct in both regards -- as Terry's comment points out, an undisturbed pressure altimeter's reading (imagine one sitting on a bench outside the local hangar) will change with the local barometric pressure.  TSO'ed "sensitive altimeters", i.e. ones with a hundreds pointer, not just a thousands, are required to have a knob and window, called a Kollsman window, for setting the local barometric pressure, or standard pressure (29.92) when above the transition altitude (18,000' in the US, check with the local aviation regulator for where you are in the world).  "Glass cockpit" systems have a knob/function for the same job -- whether it be a dedicated knob, or a mode on a multi-function knob.

Can you define TSO'ed "sensitive altimeters"? Not all of us have been through flight school.

@FreeMan "A TSO is a minimum performance standard for specified materials, parts, and appliances used on civil aircraft. When authorized to manufacture a material, part, or appliances to a TSO standard, this is referred to as TSO authorization. Receiving a TSO authorization is both design and production approval." That's from https://www.faa.gov/aircraft/air_cert/design_approvals/tso/, and TSO stands for Technical Standard Orders.

@Terry -- perhaps we should have a Q&A here explaining sensitive vs. non-sensitive altimeters? (I suspect that many people who've been through flight school did so without ever seeing a non-sensitive altimeter!)

@UnrecognizedFallingObject Be my guest. I'm a lazy old man. LOL. One might also ask about the additional altimetry requirements and checks for using 1,000 foot separation in the North Atlantic MNPS. Again, somebody else should ask it as my experience is from the 1990s and I'm not sure it's applicable anymore.

@UnrecognizedFallingObject Ask and ye shall receive! (or someone else will ask - either way really)

Yes - as noted above, an Aneroid Altimeter will indeed show different altitudes if you come back the next day.
and No, a Radar Altimeter will not change its reading as it is measuring the altitude by "listening" to return echos from its transmitter.

Would a radar one change altitude based on what's below the plane (mountains, buildings, etc.)

@Alexus - Yes, this is one reason they exist, to provide very accurate "AGL" altitudes for pilots flying at low altitude or through high terrain. It was initially developed by the military as "terrain-following radar" for low-altitude bombers and strike fighters like the B-1B and F-15E, and a simplified version found its way into commercial airframes as the "Terrain Avoidance Warning System" or TAWS.

You mean on airliners there would be 2 systems side by side?

@Alexus -- yes, airliners carry both barometric and radar altimeters -- the latter provides increased precision during instrument approaches in low visibility as well as helps with terrain avoidance.

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Why is this plane flying in such a tight circle?
I was in a car in NYC on 2/18 going from Queens to Manhattan. While going over a bridge I noticed some (what looked to me like) very tight circular contrails in the sky.
The plane was still there but very high up and/or far away and I couldn't make it out.
The photos here are from my phone, through the window of a moving car on a bridge, so not the best, but it's all I could get.
I never noticed anything like this before and I was curious what this might be.

Military exercise?

@757toga it's hard to tell from the photos (especially the thumbnails) but the line coming out the arc on the right didn't seem to be at all related, and it otherwise did not look like skywriting (I have seen that before).


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Is it harder to learn to fly rotary wing after learning on fixed?
I have a private license for single engine land and I want to transition to helicopters. I have been told by some that learning helicopters is more of a challenge after learning fixed wing. However I have yet to see any good sources or explanation for this. Is this generally true and if so why?

This is not opinion based.  Although  I can't find authoritative sources, it is widely accepted that fixed wing to rotary is harder than the other way around and there are a couple of potential killers if fixed wing pilots handle helicopters like they would a fixed wing.  Any instructor rated on both will tell you that,  I only have about 10 hours total in fixed wing so nowhere near enough to answer but this is a valid question.

...and please, no answers or comments about "they are both the same except when hovering" or any such nonsense.

Related: What are the drawbacks of being a helicopter and fixed-wing pilot at the same time?

@Simon, I think you basically answered the question already with the comments and your answer to the question linked by mins.

AOPA has a series of articles where a fixed-wing pilot learned to fly a helicopter. One of his lessons learned supports the idea that it's harder to go from fixed-wing to rotary:

There is a negative transfer of learning from airplanes
All my aviation experience prior to flying helicopters involved a sort
  of building-blocks approach. Each new skill was based more or less on
  the previous skill. Additional class ratings simply came down to
  differences training. Such is not the case with helicopters. In some
  cases, fixed-wing knowledge is a hindrance. This is especially true
  with low-G mast bumping in the Robinson. Most poignant for me was when
  I was maneuvering for a steep approach and I saw the airspeed bleed
  off fairly quickly. My first thought was "stall!" when I was actually
  right where I needed to be.

But if you read the whole series, he does mention transferable skills like radio work, regulations, navigation etc. that are still useful, so while maneuvering the helicopter may be more difficult for a fixed-wing pilot at first, that isn't the only consideration in learning to fly one.
There are lots of opinions out there about transferring from fixed-wing to helicopters, but there doesn't seem to have been any systematic analysis or investigation about it. As far as I could see, the FAA's Helicopter Flying Handbook doesn't say anything about transitioning from fixed-wing aircraft so either they didn't consider it at all or they did but decided that it wasn't a significant issue.

Rotary flying is much harder than fixed wing to learn, however learning after you get a fixed wing license is easier in some ways because you already know:

Radio procedures
How to set up and use instruments
Navigation and flight in controlled airspace
Air law and regulations
The "gestalt" of flying

This means all you need to do is learn to fly the helicopter, which is tricky because helicopters are much less stable than fixed wing airplanes. If you take your hands off the controls of fixed wing airplane it will generally stay pointed where you put it, especially if it's trimmed properly. If you take your hands off of a helicopter's controls in hover things will go very, very badly in a real hurry so you have to be making constant control inputs. The controls of a helicopter are also very sensitive, it is a very different (and more challenging) type of flying.

This is contrary to the industry opinion that fixed wing->rotary is harder than rotary->fixed wing.  I am still looking for some good reference but in essence, fixed wing pilots have some ingrained habits that can be really bad in a helicopter.  It seems that rotary pilots don't have them during ab-initio training and don't seem to pick them up in the transition.  the points you make are valid, and would reduce the number of hours needed (indeed, are credited, at least in the UK where the minimum hours is reduced by 5) but not how to safely fly.

In the UK having a fixed wing also means you don't have to do a qualifying cross-country as part of your rotary license.

It's an entirely different kind of flying, altogether!

I am serious, and stop calling me Shirley.

I think as a fixed wing pilot you will will have gained fixed wing "habits" that are not transferable to helicopters.
The main one I can think of is if your engine fails on a fixed wing, you may pull to reduce the speed to best glide and gain a few hundred feet of height. This would be a disaster in an helicopter.
No doubt an helicopter pilot (of which I am not) would tell us that possibly in high speed cruise it might be possible to lose some forward speed and translate this into height, but what I am saying is that the primary consideration in an helicopter is not to lose rotor speed because the blades once stalled cannot be un-stalled (they are at the back side of the drag curve), even assuming they are undamaged at that point. So in an helicopter you drop the collective immediately to preserve rotor speed.
I once heard somebody say that you needed 500 hours in helicopters if you were experienced in fixed wing, before that guy would let his grandchildren fly with you.

HI Phil.  Helicopter pilot here.  Even at cruise speed, trying to trade speed for height will kill you.  If you leave cruise pitch where it is, you have about 2 seconds at most to get the collective down.  Pull and the rotor speed will be unrecoverable in less time than you could change your mind.  What most people don't realise is that it truly is unrecoverable.  Even if you could get the engine going again, it could not overcome the drag of the stalled blades but even if it could, they would have folded up, "clapping hands at the top" and will be flopping around uselessly.

Hi Simon The reason why I added that comment was that I'd heard of a turbine helicopter running out of fuel at low level at about 150 knots and nosing in so I just wondered - maybe its lower the collective to keep the rotor speed but use the cyclic to keep the nose up???? As related in my post I have never even sat inside a helicopter, all my experience is fixed wing. If I was going to fly a helicopter I would want one with heavy blades which would make it even more expensive I guess. Regards Phil

Without knowing the details, impossible to say, especially without knowing the height.  150 feet or higher and it should have been survivable.  Take a look at the height velocity "dead mans curve" for different helicopters.

As a fixed-wing pilot who transitioned to rotary, I would agree that the actual "stick and ruder" -- or "cyclic, collective, and directional pedals" -- part of flying rotary wing is harder than flying an airplane.
First and foremost, there are actually three different kinds of flight in helis:

hovering flight
forward flight
autorotation

and each of these three types of flight needs to be learned individually, has its own muscle memory, and its own set of gotchas.
Secondly, helicopters are unstable, meaning they must be flown hands-on at all times, and helicopter trainers (e.g., Robinson, Engstrom, Schweitzer), in particular, don't have stability augmentation systems, precisely so that the pilot learns to control the helicopter with the controls.
Third, and this is more intuitive than evidentiary, helicopter aerodynamics are much more complicated than airplanes. Trainer nose-wheel airplanes can be flown quite lazily, without much rudder input, and with unsubtle control input, and not have too much an effect on the flightpath or safety of the aircraft. In a helicopter, every single input on one control (cyclic, collective, throttle, pedals) must be fully compensated for on every other control, to keep the aircraft from deviating from its flight path.
Fourth, the controls on a helicopter are much more sensitive than on an airplane, and must be handled with the utmost of care. The operative phrase for handling the controls is to "milk the mouse," meaning to use very small control control inputs.  (My own instructor, when I was unsubtle on the cyclic would joke "Hey Bob, are you making salad?")
Those are the flight control issues, but there are a few other operational issues with helicopters that make them more difficult:

helis often operate off-airport, including confined area and pinnacle operations, operating on slopes, operating on unimproved (or even unlandable) surfaces, all of which require specialized skills
helis often carry external loads, which adds an additional handling complexity

nice answer. I can see learning heli is much harder than fixed wing. But the OP ask for learning heli after the fixed wing. Your answer is great but also true whether learning fixed wing first or heli first.


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Where does the FAA define the dimensions of Terminal Radar Service Area (TRSA) airspace?
The airspace over the United States is classified into various areas of "alphabet" airspace (A, B, C, D, E, G). This is known as "regulatory" or "rulemaking" airspace because the airspace classifications are established by rule and only after a public comment period. The regulations defining this areas of airspace are at 14 CFR 71, which "incorporates by reference" the airspace definitions published in FAA JO 7400.11 "Airspace Designations and Reporting Points".
There is also "non-regulatory" or "non-rulemaking" airspace, which is established without a comment period. TRSAs are one example of non-regulatory airspace. By definition, the definitions for these airspace areas are not published in the Federal Register or Code of Federal Regulations.
FAA JO 7400.2 "Procedures for Handling Airspace Matters" says that "Nonrulemaking actions must be published in the National Flight Data Digest (NFDD) on or before the applicable charting cutoff date." The NFDD is a stream of changes to many aspects of the NAS, without any clear historical organization.
Both regulatory (prohibited and restricted) and non-regulatory (military operations, alert, warning, and national security) special-use airspace areas are listed in FAA JO 7400.10 "Special Use Airspace". The actual definitions are published in the Federal Register (for regulatory SUA only) and the NFDD. TRSA airspace is not SUA and is not listed in the 7400.10.
Is there a published document which compiles the definitions of any or all TRSA airspace?


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If all control surfaces were disabled, could a passenger plane be controlled by distribution of the passengers?
Let's assume that a large military plane has its control surfaces disabled in combat. The engine(s) can still be controlled. There is a substantial number of ground forces being transported.
In theory could the plane be flown purely by distribution and redistribution of the weight of the passengers? Note these are military personnel and are used to acting on orders.

If the plane would still fly relatively stable on its own, I guess the reasonable thing would be to only use the engines to control the aircraft, like they did with OO–DLL.  CoG shift through passengers would be too sluggish imo.

Check out this question: What alternative methods are there for controlling a plane when flight control surfaces have failed?

In theory, yes, in pitch anyway, and to some limited degree.  That's how hang gliders work.

There are different degrees of control. The plane could at least be made to crash in a different debris pattern than if the input was not made, and even that's a form of control.
As the whether the plane could be controlled enough to land, and do so, as the question states, with all of its control surfaces disabled, let's see. You get considerably pitch control authority, with poor speed and accuracy, but very little direct roll control. Differential thrust provides a decent amount of yaw control.
So the first question is: Is the plane already on the glideslope of a straight-in approach to an airstrip? If yes, then passenger movement should, with some luck, suffice to get the plane down to the ground.
If not, getting a plane to a suitable landing spot becomes more difficult. Many military transports are designed for rough field landing, so over flat and easy terrain, it should be possible to get some sort of landing that is better than an uncontrolled crash. If the terrain is not so forgiving, it comes down to trying to navigate and fly with greatly reduced control responsiveness and precision, where many inputs will be made wrong, and down to whether they can be corrected in time.
To make an analogy, if a driver transporting a blind passenger falls out of the car, can it be controlled from the back seat by moving controls with the passenger's walking stick, acting on advice shouted from cars passing by? That's how trying to fly a plane that way would probably feel.
That said, much stranger things have happened. People have fallen out of airplanes without parachutes and survived. Planes have landed themselves smoothly after the pilot has ejected. And landings with a loss of most controls have happened as well.

Why no direct roll control? Lateral CG shift has no influence?

@Koyovis Well, you could get a bit of a shift. It would be very slow control, though, compared to the authority in pitch.

"The plane could at least be made to crash in a different debris pattern than if the input was not made..." Excellent wordsmithery!

"Direct roll control" is not needed to guide a stricken aircraft.  Climb/descend with changing thrust. Turn with differential thrust (dihedral will help couple in some roll).  Control speed with pitch (If you can). Moving passengers side to side probably will not work well, but only roll stability is needed to keep aircraft upright, especially if lucky enough to "lose all control" with a trimmed aircraft flying straight and level.

Let us consider the longitudinal stability
In a longitudinally stable aircraft, any wanted or unwanted increase in angle of attack will cause the pitching moment on the aircraft to change so that the angle of attack decreases. Similarly, a wanted or unwanted decrease in angle of attack will cause the pitching moment to change so that the angle of attack increases.
The above stability is possible provided the aircraft has a stable centre of gravity
Modifying the CG by moving the passengers to modify the angle of attack is by definition playing on the instability boundary of the pitching moments, this is possible only with extremely reactive commands, that is with FBW (fly by wire) and not with FBP (fly by passengers).

A0A equilibrium will have a new neutral point after the passenger change position.

Passengers  change position I would say no.   Robotic bodies fly by wire controlled why not!

Shifting the center of gravity by sending people fore or aft was once common practice on large airships to change pitch, taking advantage of a lever arm of several hundred feet.
It is theoretically possible to do this on a passenger airliner.  Altering the CG with a fixed (frozen) elevator trim will have the effect of speeding up or slowing down the aircraft trim speed.  Moving weight forward means elevator trim (aerodynamic) needs more airspeed to raise the nose (increase the angle of attack of wing), causing the plane to climb and lose speed, eventually settling into its new trim speed.
Moving weight back makes it easier to raise the nose, so the speed stability mechanism (elevator trim) raises the nose at a lower airspeed.
The key here is realizing the limitations of this technique.  If the weight is too far back,  the plane will not pitch down and gain airspeed before stalling.
A practical application of moving passengers would be to help flare the aircraft just before landing.  Naturally, a little practice at altitude would help.
For a stricken aircraft, control by differential thrust has been demonstrated.  Moving weight fore and aft could be tested.


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More than one runway with the same heading. How are they labelled?
Given that runways are labelled by magnetic heading. What happens in airports that have more than one in the same direction?
How are they labelled?
Are there any such airports?

I'd guess that the great majority of airports with more than two runways have at least two parallel. Not all, since triangular arrangements are quite common (especially at former WWII-era air bases). Probably almost all airports with more than three runways have at least two parallel.

Most major commercial airports have at least two parallel runways, even -- so this configuration is far more common than the OP envisions.

Voting to reopen this, as the marked duplicate question does not cover the case of more than 3 parallel runways

@J.Hougaard - last paragraph of this answer covers 3 runways and one of the comments covers 4-6 runways. (I know... comments can be deleted.)

Left, right and centre.
For example, Heathrow has 27 left (27L) and right and 09 left (09L) and right.
See here for the airfield chart.
You can see the numbers close to the end of the runways.
Here's an example for an airfield with a 34 left runway.

If there are more than three runways more or less parallel, one or more runway numbers will be shifted by 10 degrees, e.g. 08L, 08C, 08R, 09R.
From the Wikipedia article on runway numbering:

At large airports with four or more parallel runways (for example, at
  Los Angeles, Detroit Metropolitan Wayne County, Hartsfield-Jackson
  Atlanta, Denver, Dallas-Fort Worth and Orlando) some runway
  identifiers are shifted by 10 degrees to avoid the ambiguity that
  would result with more than three parallel runways.
For example, in Los Angeles, this system results in runways 6L, 6R,
  7L, and 7R, even though all four runways are actually parallel
  (approximately 69 degrees). At Dallas/Fort Worth International
  Airport, there are five parallel runways, named 17L, 17C, 17R, 18L,
  and 18R, all oriented at a heading of 175.4 degrees.
Occasionally, an airport with only 3 parallel runways may use
  different runway identifiers, for example when a third parallel runway
  was opened at Phoenix Sky Harbor International Airport in 2000 to the
  south of existing 8R/26L, rather than confusingly becoming the "new"
  8R/26L it was instead designated 7R/25L, with the former 8R/26L
  becoming 7L/25R and 8L/26R becoming 8/26.

And left and right are from the point of view of a plane coming in to land:  a runway that's labelled 09L at one end will be 27R at the other. Also, there might be "missing" runways because something was decommissioned. Chicago O'Hare, for example, has runways 10L/28R and 10C/28C but no 10R/28L.

Note that only the runway number is different in the case of more than 3 parallels. The runways themselves are not actually shifted 10 degrees. Also, usually in the case of 4 runways, you'll have R+L of one number and R+L of the other. Atlanta is an example of 5, with 08R/L, 09R/L, and 10.

@DavidRicherby - in O'Hare's case the southernmost runway has been labelled C because 10R/28L was planned but never built. They finally got around to it in October 2015.


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What is the difference between CTAF and the Tower frequency? Why does the Chart Supplement list the same frequency twice?
At many part-time towered airports the Chart Supplement will have a listing along these lines:

COMMUNICATIONS: CTAF 119.2, TOWER 119.2 (1400–0500Z‡)

The CTAF is the same frequency as the Tower. What does this mean? Would it be correct to say that we "Contact tower on (or using) the CTAF"? Why or why not?

Question inspired by discussion here. See also: When flying to a towered airport when the tower is closed, what frequency should I use?

The same frequency is listed twice because the services provided on that frequency are mutually exclusive. When one service is available, the other cannot be.
When the airport has an operational control tower, airport traffic control service is available on the Tower frequency:

AIRPORT TRAFFIC CONTROL SERVICE- A service provided by a control tower for aircraft operating on the movement area and in the vicinity of an airport.
(See MOVEMENT AREA, TOWER, ICAO term AERODROME CONTROL SERVICE.)

This service is provided to ensure safe spacing between aircraft operating on the airport surface, in particular the runways. Some airborne separation services may also be provided between IFR/IFR, IFR/VFR, and even VFR/VFR traffic depending on the airspace classification.

When the airport does not have an operational traffic control tower, this service is not available. Instead, pilots use the Common Traffic Advisory Frequency to self-announce their position and intentions, which allows operations to be carried out in a safe manner.

COMMON TRAFFIC ADVISORY FREQUENCY (CTAF)- A frequency designed for the purpose of carrying out airport advisory practices while operating to or from an airport without an operating control tower. The CTAF may be a UNICOM, Multicom, FSS, or tower frequency and is identified in appropriate aeronautical publications.
(See DESIGNATED COMMON TRAFFIC ADVISORY FREQUENCY (CTAF) AREA.)
(Refer to AC 90-66, Non-Towered Airport Flight Operations.)

This is not a separation service, but a frequency provided to aid pilots in their see-and-avoid and safe sequencing responsibilities. The Federal Aviation Regulations still apply and right-of-way rules must still be followed, even if an aircraft is not transmitting on the CTAF.

At many airports with part-time control towers, the main Tower frequency and the CTAF share the same frequency, indicated on the Sectional like this: CT - 118.6 ★ ©.

But this does not imply that they share the same function, which is why the Chart Supplement both frequencies separately. The star symbol indicates that the Tower frequency is only in operation some of the time, and when it is not the "C" symbol indicates that the CTAF is 118.6 as well; when the Tower is open there is no CTAF. This configuration is very common at fields with part-time towers but it may not always hold true.
So it would be incorrect to say that you "contact the tower on CTAF."
Instead you contact the Tower on the Tower frequency, and utilize the CTAF if the Tower is closed.

Note that using the same frequency for Tower and CTAF has an important benefit that aircraft flying in the airspace at the time tower is opening or closing are implicitly switched over so noone calls the wrong frequency because they have their clock a couple of minutes off or just didn't get around to switching the radio yet.

The chart supplement also helpful explains in the airspace section that there's "CLASS D svc 1400–0500Z‡; other times CLASS G".


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Can the 747 be re-engined as a twin?
Since more and more airlines are dropping the 747 for the 777 or equivalent due to fuel efficiency, a theoretical question came to mind.
What do you think is preventing Boeing from creating a 747 twinjet (two engines instead of 4).
I know there are probably design/engineering issues associated with such a conversion, but I reckon that if it was done it may increase sales to go up against the A380? Not sure the math as well, but perhaps the engine from the 777 could be used or configured for higher thrust?

What do you think is preventing Boeing from creating a 747 twinjet  because it makes no sense and there is no demand?

Do people ever ask "Why don't we put a BMW engine inside a Kia"?

@curious_cat you would be surprised what people actually ask ( and do )

The A380 hasn't made any sales for well over a year. The market for such an aircraft is quite small.

@Federico: Fascinating!

@curious_cat Poor analogy. The question is actually more like, "Why don't BMW make their cars available with different engine options to suit different segments of the market?" To which the answer is that, of course, they do.

Currently, the 777 has engines that have a max thrust of 115,000 lbf, for a total of 230,000 lbf of thrust. The 747-8 has engines with a max thrust of 66,500 lbf, each, for a total of 266000. And just to throw in for comparison, the A380, currently has engines each producing 72,000 lbf of thrust, and a total of 288,000 lbf.
Right now the 777 has some of the highest thrust producing engines in commercial service. This does not take into account the extra thrust that is required for engine out issues. With a 2 engine design you have to have enough extra thrust for loss of engine during take-off at max take off weight. Whereas with a 4 engine design, you only have to have enough thrust to account for loss of 1 engine. So for a 747 twin engine aircraft, as a rough estimate you'd need to have each engine produce at least 150,000 lbf of thrust if not more.

So in theory, if they revised the 777 engine to achieve 150,000, it could still be an option to keep the queen alive?

If they could get an engine designed and built to have the necessary performance, it could be built. Also, realize the 777 engine is about the same diameter as the 737s fuselage, so scaling up to the needed thrust for a 747 would be larger, and would entail a lot more design changes and rescaling of the wing and gear of the 747s design. Is it worth it? From what I can see, no, it just couldn't be redesigned in an effective manner for an appropriate cost.

Don't forget that the B747 was designed almost 50 years ago. If you tried to make that sort of design change (and assuming one of the engine manufactures was feeling crazy enough to try designing something with 150,000lb thrust for it) , it would be like your grandfather's army knife that is still the same "knife," despite having new wings, new landing gear, a new fuselage, a new cockpit, and a new tail. If it was a remotely sensible commercial programme, you might as well also change the only thing left from the original - i.e. the name.

"Right now the 777 has some of the highest thrust producing engines in commercial service." Not just some of. The GE90-115b (the 777-300ER engine) holds the world record for thrust produced by a commercial jet engine (and I'm not aware of anything in the works that is likely to exceed it.)

@alephzero: Granted, it wouldn't be the first time Boeing's done something like that - look at the 737 MAX, which has basically nothing in common with the 737-100 except the basic configuration.

For a wing design it's better to have more engines because the loads evenly spread along the wing. This allows a lighter wing design.
In addition you must provide a minimum yaw control ability at one engine in-operating during take-off. This means you have to enlarge the vertical stabilizer. This enlarges the drag during normal operation.

But isn't it also true that the engine load torgue further out on the wing increases loads?  I might have to have less strong engine MOUNTS, but the wing is till taking on the same approximate load (20K lbs for GE90 on 777 vs 12K lbs x 2 for GEnx on 747)  accounting for thrust, you still have to engineer the wing root and inner wing for 110K lbs of  thrust.

Yes. But take the impact of the worst case into account for this calculation. If one structure fails... this leads to different safety margins. So it isn't that easy to compare this only by taking mass and thrust related loads. Additionally the engines you compare are from different age: 95 to 04. So I would say the thrust to mass ratio is not compareable.


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Why the inward engines-placement on the An-124?
I am curious as to why the engines (specifically engines 1 & 4) are positioned far closer to the fuselage on the An-124 compared to a typical 4-engined jet.
I am curious as to whether this design was totally due to wing aerodynamics, or if it may also be related to the shoulder wing design & wing droop when not flying.

Overlay that matches the wingspan of a 747-100 (top) to an An-124, showing the more inward placement of the jet engines.

@ymb1 no the edits made it WAY more clear! Thanks!

It might actually be a closer comparison to the 747-100 through -400. That image is of the -8, which has a different wing.

In general, designers would like the engines to be closer to the fuselage, more like the An-124. With the low wing on the 747, the engines have to be placed far enough out that they can still have enough clearance from the ground. This trades with the height of the landing gear and dihedral of the wing at the fuselage. With a high wing on the An-124 they have plenty of room and can place them closer.
Although the engines do provide some relief to the regular bending loads from lift on the wings, there are lots of reasons to keep the engines closer to the fuselage. Better handling with engines failed is one. But it also affects the amount of wiring and tubing you need to get to the engines, and there are other loads like hard landings that are easier to design for with the engines closer to the fuselage.

In that case, why are there not more airplanes designed with engines closer to the fuselage?

Rest assured that the existing engine placement was arrived at after lots of studies. This answer gives some reasons. Generally, placing them further outboard enhances the bending relief effect, and ground clearance can be achieved with raising the engine like on the 777 or newer 737s.

@user6035379: If you stop reading answers after the first three words, you will never learn why. Too bad!

Just a guess: The requirements asked for take-off with two dead engines on one wing. Not fully loaded, but empty except for some fuel to transfer to a safe airbase. The An-124 was developed as a military transport and can operate from improvised airfields. Such a requirement would make sense.
The same requirement existed for the An-70 turboprop transport, but here the inboard engines had to be moved far enough out to allow paratroopers to jump off the plane from the forward doors without ending up in the props. So it is very likely that the same requirement was asked of the An-124.
Why engines are where they are was discussed here.

@ymb1: Call it as you like: More rudder authority, less yawing moment due to engines, smaller vertical to fulfil all requirements.

There's also the structural issues, balancing the bending moments of the lift with the engines when in the air and making sure the wing can support the weight of the engines on the ground.  Not caring about noise would also mean engines could be closer.

Also the high wing configuration of the AN-124 would better facilitate the engine closer to the fuselage; a similar configuration exists with he Lockheed C-141 and C-5 aircraft as well.  So too, does Boeing's C-17 and the Illyushin IL-76.

@Notts90: All structural issues are identical for airliners, too. Noise is taken care of by the low wing layout of airliners. No, this does not explain it.

I stopped reading your answer after the first 3 words 'just a guess'.


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What are the constraints and procedures regarding IMC formation flight?
"Instrument Meteorological Conditions" are those in which flight under Visual Flight Rules is not permitted. This changes based on airspace but for the most part IMC means that flight visibility is lower than either 3 SM or 5 SM.
A "standard formation flight" is one in which all elements maintain no more than 1 NM laterally and no more than 100' vertically from the flight lead. Clearly a standard formation could be carried out under IFR if flight visibility were lower than 3 SM. But is there a lower limit adhered to by any organization? Demonstration teams are often said to fly with mere inches between their wingtips; visibility would have to be extremely limited to make that impossible, albeit much more risky, or so I imagine.
Furthermore, do all cloud types and instances generally restrict visibility the same amount? Or is there variability between types, or even intra-type on different days?

Inspired by https://aviation.stackexchange.com/q/96769/54557

Short answer:  The constraints are that you maintain sight, and procedures are to train and practice regularly, and thoroughly brief all contingencies.
CONSTRAINTS
Visibility can vary significantly in different cloud types, and even within a layer.  Cumulus are generally the most dense and therefore have the poorest visibility.  They are generally the most turbulent as well, which makes compensating by flying closer more of a challenge.
That said, there is no real lower limit or minimum visibility for scheduling operations as it would be impossible for a ground station to measure or forecast.
The reality is that the lower limit is established by the comfort level of the wing pilot.  You tuck it in as close as you are able to when the clouds get thick, and for several minutes your entire world may shrink down to the immediate and life-affirming task of holding a small airfoil shaped wingtip or nav light in a constant relative position on your canopy.  You might be doing this while getting bounced around and having lost your other positional reference points because the clouds are so thick you can no longer even see the fuselage of the lead aircraft.
This is where a having an NFO, (Naval Flight Officer) with a soothing voice in the right seat is worth its weight in gold.  While you fight to keep from either losing sight or hitting the other plane, your situational awareness is enhanced by little sugar calls on the intercom like "in an easy right turn to base leg now, we should be rolling out in about 10 more degrees... passing 4000 for 3000 at about 800 FPM down..."
Sometimes that wingtip will even flash in and out of sight briefly as you transit the thickest sections, anything more than about a second or so should trigger execution of lost sight procedures, which takes me to...
PROCEDURES
Lost sight under IFR is an emergency because you suddenly have an aircraft, (the wingman) operating in IMC, without an IFR clearance, WELL inside of ATC separation requirements.  So, you can’t count on ATC for the immediate separation needs, you need to take action.
Lost Sight procedures are briefed on every formation flight, and they need to be deeply ingrained and automatic so when that small piece of metal 10-15 feet away, (that's attached to a much larger piece of metal 30-40 feet way) suddenly disappears from view there isn't any confusion about what to do.
The important thing is for the lead to remain predictable and smooth, continue to comply with the IFR clearance, and for the wingman to take action.
Specifically:

Climb or descent - Lead continues to assigned altitude, wingman adds power (if in a descent) or pulls power (if in a climb) to level off.
Turn away - Lead continues the turn, wingman levels off to achieve a heading split.
Turn into - Especially important for the wingman to call "lost sight" on the radio, and over bank to get away.  Lead should level wings until a 30 degree heading split is established, then resume turning to assigned heading if being vectored.

There are additional techniques that may be covered in more detail depending on the circumstances, and/or experience levels of the crews or number of times they have flown together.
For example, if I was the formation lead pilot and was briefing with a new pilot/NFO crew I hadn't flown with before I might add some mission specific details like "there's a pretty thick layer on the way to the MOA, if you lose sight in the climb call it out on the inflight freq and I will keep climbing, calling every thousand feet as I pass.  Get on the radio with ATC and pick up your own squawk, and ask to resume climbing once we have at least a thousand foot split.  I will let you know as soon as I find VMC, and audible a rendezvous plan on the inflight freq."
Those who fly together regularly start to think alike, and you would likely spend 3-5 seconds reading the bulleted points above straight off the mandatory items on the briefing card, and then move on with the rest of the brief.

Two very different types of IMC formation flying that I had some experience in, with fairly different criteria.  Both of them in the military.
First is the very close formation -- what we called "fingertip" formation in the training command, which we flew in T-37's and T-38's, and basically what you see when the demonstration teams are tucked in tight.  While the specifics would differ from airplane to airplane, the basic idea is that the #2 wingman is slightly aft of lead, and his cockpit is probably within one wingspan of lead's fuselage (i.e. the wingtips overlap in left-right aspect, but are separated fore-aft and up-down).  When you consider how thick clouds would have to be to not see from one side of a fighter or trainer aircraft to the other, especially when lead has some lights on in the most useful configuration, that's not common.
The other place that this sort of formation can be useful is in air refueling... the receiver is behind and below the tanker, but within the length of the boom (once hooked up), and there are lights on the tanker's belly.  While there are procedures for lost-sight in all of these cases (lead goes faster & up, receiver or #2 goes slower & down, briefed turns in many cases, more complex for a larger formation), with experience it becomes reasonably common for a 2-ship or a 4-ship to fly around in IMC, for all purposes as a single aircraft.  The "standard formation" definition has meaning for ATC, but for the formation itself you're in fingertip, or something with slightly more space (probably not used in IMC), or some tactical formation (much more space, wingman can pay attention to things beyond just holding an exact position on lead), and whether or not it is "standard formation" is an afterthought.
The other type of formation that works in IMC is what in the C-130 world we call SKE, for Station Keeping Equipment.  The basic technology is really old, and is more-or-less TCAS version 0.1 between cooperating aircraft only.  It used dedicated equipment, not transponders, and had a whole host of parameters & procedures & some limitations (especially with large formations), but allowed us to fly at 4,000' trail and co-altitude.  That very quickly (at least once you got beyond one wingman) became non-standard formation for all ATC purposes, and was a long string of aircraft in the sky.  You'd follow lead, he gets a vector, and when the wingman got to the same point in space, he makes the same turn.  We could fly a formation ILS that way; once you intercepted the localizer, each aircraft flew the ILS course & vertical path, but kept spacing on the aircraft ahead.  It was ponderous, but it worked.
The current version of SKE, from all I've heard, is a world ahead of what we had in the 1990's and early 2000's: far better datalink, integrates with the autopilot much better, etc.  Pretty cool what you can do with 2020-era digital technology compared to 1970's era stuff!
Other aircraft could do something similar using air-to-air TACAN or air-to-air radar.  I'll let somebody more familiar with those procedures add details.
But in both the case of fingertip formation and SKE formation, you could fly an ILS down to minimums, and land the whole formation on the same runway in one pass... the "wing landing" might get interesting in really low vis, but single-seat jets don't do Cat II or III, so nothing crazy low by the time you're on the runway.  The SKE formation landing ended up with (IIRC) about a mile between aircraft, so you could have more than 1 on the runway at once, but there were procedures to make that work, and it generally did work quite well.  Equipment getting temperamental was the most common issue.
Good times, but not really anything I feel the need to get back into with the 737!


text:
How to find accurate civil aviation flight traffic data?
I need to review civil aviation flight traffic data for several regions around the world.
My preference is real-time data, but data delayed by 5-15 minutes is fine.  Historical data (going back 1 year) works too, but is not quite as useful.
Ideally, I would like the data to include the aircraft type and registration number.
I'm not getting compensated to contribute to this project, so having the data be free is important.
I am aware of 2 websites that provide this information for free:

Flightradar24
FlightAware

The challenge I have with those 2 sites is that the data appears to be incomplete.  Aircraft will be shown on one site, but not the other.  To make matters worse, some aircraft are completely missing from both sites.  I've noticed this, especially, for helicopter traffic (including near airports).  Including civil helicopter data is essential.
What other resources are available for this civil aviation flight traffic data?

"Resource location" is specifically off-topic

One such site is
https://globe.adsbexchange.com/
As for the problems you mentioned, I'm sure this one has them too, but adsbexhange does seem to provide a lot of data per target. It also has convinient and extensive filters availlable for free. I do not know if there is any history -function on this site.

Your recommendation is a big help.  Upvoted.  Thank you!


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What's the red light in the left wing of a N.A. T-6 for?
There's a yearly flight show near my location, and there's always this North American T-6 (D-FUKK):

Today, I noticed that there's a single white light in the leading edge of the right wing, but a white and a red light in the left wing.
What's that red light for?

@mins and there is, but he's asking about the one on the leading edge ;)

lol that airport code


text:
What's that pipe at the exhaust of this engines?
On my last vacation, I took lots of photos from landing aircraft, and here are two showing the engine of two B737:

What is that pipe on the cone of the exhaust for?
I also saw them on other, larger aircraft.
Often, the exhaust looks like a large, black hole where you don't see anything inside. Maybe, those engines have such pipes, too, but you can't see them.
My guess is it's for cooling of the shaft or similar, as the exhaust gases should create a vacuum in the pipe.

Exact duplicate of a question asked yesterday.

Its there basically to vent air out of the oil pump

Quite interesting. I have this question in mind since some time, but didn't notice it was asked yesterday...

Never seen it happen before. Whats also weird is the 2 pictures that have different shaped exhaust nozzles, but its a slight difference.


text:
How (often) are jet engines cleaned?
In the on-board magazine of my today's flight, there was a report on how fuel efficient the airline flies. They accomplish this by new scitimar winglets,  daily optimized route planning, and regular cleaning of the jet engines.
I never heard about this cleaning. How and how often is it done?

Related video

One of the top causes of cabin smoke/stink incidents is failing to do a sufficient post cleaning engine run to purge the detergent from the compressor before dispatching.

What's the detergent used?

It used to be done launching crushed walnut shells into the running engine and the blades are polished by the process.  Known as Coke Blasting.  The process is uncommon these days.
Similarly they use a walnut shelll coated emery cloth to polish crankshafts after machining
http://www.dennisdawson.com/jetblast.htm
It is still commonly done on industrial turbines apparently


text:
Why does KBOS have such a short runway?
Runway 15L/33R at Boston Logan is only 2500 ft, which is barely long enough for a Cessna, let alone an airliner. I’m wondering if anyone knows why this runway was built or what purpose it serves. I find it a bit odd considering KBOS is certainly not intended for GA use. I don’t think there are any other class B airports in the country with such a short runway.

"It's not uncommon to see Dash-8s and Cape Air Cessna 402s landing on this runway. Never seen a take-off though." - https://www.airliners.net/forum/viewtopic.php?t=327293

15L/35R is actually used for GA purposes although most of the time it is just used as a taxiway. KBOS has a GA ramp and FBO just north of the International Terminal (E). It may surprise you that most of the large Class B airports still have GA traffic and the facilities to support them. Also, a fully loaded C172 only needs about 1200 feet of runway for takeoff at 40C/sea level and 650 to land, so 2550 feet is plenty of runway.

It's not quite GA, but Cape Air operates 208s at Logan (to/from the Vineyard, Nantucket, etc). Possibly they can use this runway (winds permitting) and not take up slots on the bigger runways.

Yep. There are a few large U.S. airports (e.g. ATL) where GA is, shall we say, discouraged, but there are also plenty where it's completely normal. In the cities that have Class B/C airports where GA is discouraged, there are usually one or more GA reliever airports near the city. In the case of BOS, Logan is the only thing close to the city itself. The GA airports are all out in the suburbs.

What does ATL do to discourage GA? I've flown in there several times. Fees weren't even that bad. BOS fees though... whoa!
Airports with 5 active runways (like ATL) are actually easier for GA traffic than some busy Class C airports (like MDW) or smaller Class B airports (DAL).

Besides fees, the biggest thing to discourage low-end GA is the unavailability of 100LL fuel. Looking at you, KLAX...


text:
Why can't you just set the altimeter to field elevation?
Why do you have to put the current altimeter setting in the kollsman window? Why can't you just set it to field elevation when you're on the ground? I understand why you would have to do it in the air.

I have flown into places where there were no altimeter settings or weather for that matter.  If there are a lot of traffic in the area, the pilots would agree on a altimeter setting to use to help separate traffic.  Pressure constantly changes at an airport.  It is conceivable, a plane practicing landings in the pattern may have a different altimeter setting (based on field elevation) than another plane taking off an hour later (based on field elevation).  This could cause conflicts in the air if the same altimeter setting is not used.

Because air pressure is not constant over wide areas.  Weather provides us with high and low pressure areas, and you may not know the pressure at your destination when departing.  Indeed, it may have changed en route.  OTOH, barring a major earthquake, the field elevation is not going to change significantly.
In point of fact, I DO almost always set the altimiter to field elevation when departing, mainly because a lot of my flying is from smaller fields that don't even have Unicom.  If I'm at a larger field, I might cross-check with the reported pressure, just to make sure things are working correctly.


text:
When filing a VFR Flight Plan, where/how do you include intent to use a Class B transition?
When intending to use a VFR transition route through Class B airspace, do you put the intended route on the flight plan, the alternate route (if you don't get clearance for some reason), or both?
Which fields does the information go in? e.g. Does the intended route go in route of flight and the alternate in "Other Information" or vice versa?
Example:
I am planning to fly from KHAF to KAPC. Ideally, I'd like to fly using the Bay Shore Transition, as marked on the SFO TAC:

KHAF VPSCS VPOYS VPSBT KAPC

If I'm not cleared into the Class B, I'd instead fly

KHAF SAU KAPC (staying outside the Class B)

How would I file this plan?
Where does one request a VFR transition route through class B? deals with the ATC, not the filing.

Here is the form I will be using:

Not particularly, that question is about ATC operations. I'm talking about filing a flight plan with Flight Services, e.g. through 1 800 WX Brief.

Since VFR flight plans are only used for SAR, and your route would only matter if you aren’t talking to ATC, I would file the route you intend to take if ATC denies you the B transition—or refuses to talk to you entirely.
If they do talk to you and grant the transition, then there will be a clear record of your actual path, and nobody will bother to look at (that part of) the route in your VFR flight plan. Or any other portion of your route where you have Flight Following, for that matter.
Note that, for calculating ETE and required fuel, Murphy’s Law says you should use whichever route is longest.
If you’ll be getting a WX briefing, mention the possible diversion to ensure your briefer checks that area too, but realistically, you won’t get in if there’s anything there serious enough to matter.


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Is it possible to get a digital download of the entire US VFR charts?
I'm wondering if there is a way to get a download of every US sectional chart stitched together like what you would find on Skyvector when you press "World VFR"? I tried looking on Skyvector, but couldn't find a download, and I think it would be nearly impossible to stitch together the entire US in Photoshop from the FAA downloads.

The maps are geo referenced, stitching them together should be easy.

I don't know if there's a downloadable file but the FAA does provide a "wall chart" product which covers the entire contiguous US.


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Is it safe to fly if the flaps won't retract on ground?
The Piper Archer (PA-28) i had this morning had issues returning flaps to normal after being fully extended ON THE GROUND. It required a non trivial amount of push on the air-frame itself to restore the flaps to the original position after having retracted the flaps using the stick in the cockpit.
I was told that "this is normal" and that the spring might be a little worn but that it wouldn't cause any safety issues, but none of the other Piper Archers I've flown have had this issue. I would add that it was a cold day (just above freezing) but the plane fully fully de-iced.
Is it reasonable the fly the plane?
Would you be worried? If you refused to fly the plane in that condition despite booking, would you accept the rental to be charged partially?

I would think about the outcome if the airflow for whatever reason wasn't enough to retract them. What's the density altitude like at your destination - if they are stuck down would you be able to survive a go-around?

The main thing to be concerned with, is that the spring tension is important for keeping the bicycle chain segment secure on the eccentric sprocket (item 17 below) that drives the torque tube.  If there are issues with spring tension in the return spring (item 32), I'd be concerned that slack could result in the chain potentially coming off, but that would require the tension spring on the bottom (item 22) to be not doing its job.
The top return spring should be a fair bit stronger than the tension spring, which is only there to keep the chain from going slack when the cable from the cockpit lever is slack.  There is likely a combination of a weakened return spring and some binding in the bushing blocks for the torque tube.
There isn't any risk of flap asymmetry unless the torque tube itself breaks or a control rod lets go, so a problem with the chain arrangement where it jams up or disconnects will, in a worst case scenario, just leave you stuck with flaps partially extended or at zero.
So what to do about it?  Depends.
I'm assuming they went almost fully up and you had to push them a bit to get them to move the last little bit.  If I knew the plane well, and if the panels came up almost all of the way and just needed a little nudge to overcome some friction, and they stayed up after the nudge, and every thing otherwise felt smooth and solid (when you do a walkaround, you shouldn't just be looking at stuff; you should be gently tugging and moving bits as part of your inspection to detect unusual play), it should be ok and air loads will do the job fine at retracting them as soon as you are moving with some speed, although I would snag it and expect it to be fixed the next time I saw it.
On the other hand, technically it shouldn't be doing that and if there is significant friction, or slop, or you are just uncomfortable with it, you should refuse it and shouldn't be charged for it.  A weak return spring and/or binding in the mechanism that prevents retraction on the ground is still a "defect" and needs to be rectified.
If you have access to the A&P that maintains the airplane it might be a good idea to mention it.  They may not be aware of the problem.

Thanks John, this is an awesome reply! I appreciate you mentioning how some could feel comfortable flying it but not everybody would be expected to. The flight school is forcing me to pay for the rental despite sitting it out, I think it's time to find another one.

Yeah if the mechanic inspected it and said the components were all fine and it was a little bit stiff and just needed a little help going up, I'd probably live with it temporarily.  The problem is when you don't know the source of the binding and they just ASSUME it's something benign.  Could be, or could be something more serious, like something is cracked, and the binding is an early warning sign.

I hate places that use such a policy. I understand that the school is a business, but a flight SCHOOL should't be trivializing pilot concerns. You KNOW what a proper walk-around looks like, and you correctly identified something that seemed wrong. And it was. If it were your personal plane, you'd have canceled your trip that day and have had it checked out. So you were completely in your rights to avoid using a "defective" plane. Ask them to show you in the POH/service manual the acceptable amount of play in the flaps. They're not even motorized!


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Why the excessive wait after shutdown before using the manual starter valve on the PW1000G?
Why must there be a long period (measured in hours) between shutdown and startup, before using the manual engine start valve? Nine hours seems excessively long.

I can't imagine why something has to sit for 9 to 12 hours before restarting.  I think there's an interesting question to be asked here, but it hasn't quite been asked yet--

@ymb1 Interesting thought, but this procedure says "for the safety of the ground crew", which seems separate from rotor bowing. Plus, the "solution" for bowing is to let the engine motor for some time (a minute +/-) before the FADEC adds fuel, which seems that would be possible even with manual start valve operation.  I don't have an answer here, but it seems like there will be more to it than just bowed rotor considerations alone.

The FCOM page reports MSN 7172 which corresponds to an A320neo with PW1127G engines. Why is this important? Because this engine family is prone to bowing, so much that it requires a dedicated cooling procedure on start performed by FADEC:

However, the ENGINE START VALVE MANUAL OPERATION procedure requires you to perform the MAN START of the engine, which disables many safety feature of the FADEC and does not permit the FADEC to close the start valve in case of the detection of an abnormal situation (this is actually the reason why you want a MAN START). So, I suspect it also disables bowed rotor protections, and this is why that procedure requires the engine to be completely cooled down.
If you are wondering how is possible that 9 hours are necessary for the engine to cool down, consider that such engine requires about 2 minutes of forced cooling even after 1 hour since shutdown (source). The most critical point for is around 2 hours since shutdown (source). In some forum you can find pilots reporting ~6 hours the needed time for residual EGT to go to ambient temperature for LEAP engine, which is even less problematic for thermal characteristics compared to PW.
So it's reasonable to think that the engine core with its massive thermal capacity may require many hours to reach ambient temperature, and the need of MAN START probably disable safety features that would avoid bowing, which does not allow you to start the engine if it's still hot.

Yes this is exactly it. Rotor bow persists waaaaay longer than most people think.


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Is there a known problem with the Airbus A340, fuel, flaps/slats and temperature?
From the press I read that the German minister of foreign affairs stranded with an Airbus A340 in Abu Dhabi, because the flaps or slats would not correctly retract.
To return to the airport they had to dump quite a lot of fuel first.
A German news article has a few details.
So what I read was that after the extra landing they had made a test flight, and everything worked OK.
Again, there's a German article on that subject.
However when "fueled to the top" (I guess) again, the flaps or slats had a problem to retract again.
So I'd like to ask: What detail of the construction can prevent the flaps or slats to retract when the wing tanks (I guess that's the relevant part) are filled to the top?
Likewise: What's the role of the temperature, meaning: How much does cerosine expand when the temperature rises (like percent per degree Celsius)?
Could the expansion of the fuel be the problem?

Aviation Safety Network article. A340-313, reg "16✠01"

Unsure about the reference: Should information from here get there, or the other direction? The ASN article has a few facts about the exact plane (its registration number), but my question was generic about the type of plane (A340).

Sorry for the confusion created, this is to give a link to a page which might be updated with credible elements in the future. It doesn't provide any answer at the moment of course, except the type and the aircraft registration number. Your question is an interesting one

Having a suspect that the fuel expanding with temperature could cause a physical stress on the structure, I "digged out" some numbers: aluminum alloy expands about 22PPM per °C, while petrol expands about 1060PPM per °C. So assuming the fuel comes from some underground tank rather cool, and the plane is standing in the sun, the fuel's temperature would increase by a few °C, but even for 100000 liters of fuel being warmed up by 10°C that would be "only" an increase of 1060 liters, while the tank in the plane might only expand by 22 liters. So there could be a problem...

In a A340, when tanks are full, surge/vent tanks still allow fuel to expand by 2% (equivalent to 20°C increase). Sensors in surge tanks inform the crew when they are wet. Taxi  + takeoff alone will have used about 5 tons of fuel anyway. There are overpressure protections and fuel can be jettisoned in flight if required.


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How to compare compressors of different efficiencies and pressure ratios?
I have two different engines, the CFM56, and the CFM LEAP-X. I roughly know the design point (take-off) isentropic efficiency and the pressure ratio for the LPC/HPC for both of them which are different from one another and can be provided as an input in a program.
I want to compare the compressor pressure ratio of the engines in the off-design (cruise) condition, which I believe will again have different pressure ratios and efficiencies. Can I somehow directly compare the pressure ratios of the two compressors? Do I need to have similar efficiencies to make such a comparison? Also, what role does polytropic efficiency play in this?

These engines are from 2 different era's, two different designs. CFM56 has BPR of 5-6 and OPR of about 32, the LEAP has a BPR of 9-11 and an OPR of about 40. Why do you want to compare the 2 compressors? Maybe you could explain that better in the question.

Are you sure the design point is takeoff?  I'm not a compressor guy, but I'm pretty sure the aero design point is cruise, and takeoff would be considered off-design.  Because you are at cruise for hours and hours, and takeoff for only a few minutes, so efficiency at cruise becomes the most important thing.

@DanielK In many gas turbine simulation software packages there is something called a design point, but, this should be referred to as reference point. Usually the most data we can find is on the take-off point hence the confusion of design point, it is the design/reference point of the software model, not the actual design point.


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Aircraft thrust calculation
I had a question regarding thrust calculation for an aircraft during cruise. For an aircraft flying at a constant cruise altitude and a constant Mach number (level unaccelerated flight), what exactly is the weight we use in the equation T = W/(L/D) while calculating the thrust?
What I understand is that since the mass of the aircraft is continually reducing due to constant fuel consumption, therefore the lift requirement will change. Assuming a constant L/D ratio, that implies the drag will change leading to a constant change in the thrust. Therefore the cruise thrust is not a constant value. So how can we apply the above equation for a calculation of the thrust anyway? Is it a thrust at some specific point during the cruise?

The equation refers to instantaneous thrust in unaccelerated flight. The weight in this equation is instantaneous weight.
Indeed, this weight changes over time. This is for example the reason we have to use the  Breguet range equation instead of just divide thrust by thrust specific fuel consumption to calculate the range of an aircraft.

I have seen in JD Anderson, Introduction to Flight that there is a calculation of a definite amount of thrust in the examples, where it takes a fixed weight for the calculation. Is it the weight at the beginning of the cruise phase? Maybe it is the maximum thrust required since the weight is the highest at the beginning of the cruise. And obviously, any other thrust requirement would be lower. So when we specify/if we want to specify a cruise thrust, it is always taken at the beginning of the cruise?

I think it should be clear from context. If estimating engine requirements, obviously you should be looking at maximum thrust (i.e. at the beginning of cruise). If it's about small GA planes, fuel represents a smaller fraction of total mass and doesn't change as much during a short cross country cruise. Is there a particular example you are stuck on?


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Lift distribution of a swept wing with twist, dihedral, taper
I am trying to get the lift distribution of a swept wing with taper (in chord and thickness along the span), twist, and dihedral. I know that Kuchemann Index method is used for swept wings but I am not sure how to account for the other geometric variations. The image below is what I found for a swept wing w/o twist and a swept wing with lots of twist. Not sure how many degrees means "lots". How to account for the dihedral and taper?

I am simply looking for the rough shape of the distribution so I can get an idea of where the maximum loading is and what the loading looks like around the root and tip regions. Also, is there an equation (or can an equation be derived) to get numerical values for how the lift varies along the span?

The easiest way (I believe) will be to run a simple VLM aerodynamic analysis.
For this, I suggest you use OpenVSP and VSPAERO.
Dihedral won't change the lift distribution in a significant way (in straight-ahead flight).

The lift coefficient Cl is linear with angle of attack.
Twist is extra angle of attack.
For every section of the wing dx, you're reducing lift relative to wing twist.
Schrenk's method has provisions for twist

I am trying to get the lift distribution of a swept wing with taper (in chord and thickness along the span), twist, and dihedral.

The change in spanwise lift distribution due to taper ratio, twist,  sweep angle, ... is well known and easy to find. A typical example is the following picture which shows the trend according to different taper ratios (slide number 6 from this presentation):


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Why does the 0 radial on a sectional's VOR appear not to point to magnetic north?
I have been scanning sectional charts and have noticed that the local magnetic declination is always a few degrees off from the 0 radial of VOR's around it. I have been told that the two are supposed to be the same. See image below from skyvector as an example. In this image, the local magnetic declination is 14.5 degree east. The OED VOR however has its 0 radial pointing roughly 19 degrees east of true north (which can be seen from the adjoining longitude line).

This is one example, but I see similar discrepancies all over the U.S. I have read in many places that VOR's 0 radials point to magnetic north but I have not seen many actual cases on sectionals. What gives? Possible reasons:

They do point to magnetic north. But something about the sectional is causing the apparent discrepancy
They don't point to magnetic north but they did at some point. Since magnetic declination changes over time, the VOR's 0 radial was truly pointing at magnetic north in the past but is no longer.
They don't point to magnetic north for another reason.

If VOR's aren't truly aligned to magnetic north, then it seems that going outbound on radial 090 from a VOR will not necessarily mean you are going magnetic east. And I have read on online articles and other sources that it is useful to use VOR compass roses when plotting magnetic courses. This would appear to cause errors of 5 degrees or so which seems significant.

We have a couple of questions that might help: here, here

Thank you, the second link answers my question (it's #2 on my list) and even educates me on the difference between magnetic declination and magnetic variation! Not sure how to consider my question answered at this point.


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Pitch control of tailless aircraft/flying wings with very wide elevons
I'm writing a simulation of a flying wing and I need clarification on some of its behavior.
Specifically, the simulation suggests to me that a flying wing (or any tailless aircraft) with two single elevons along the whole of the two trailing edges is not controllable. If the elevons are deflected to pitch up, for example, it generates negative lift over both wings and the torque (assuming CoG is ahead of CoL) pitches the aircraft up. But this rapidly increases AoA and therefore lift and positive torque, pitching the aircraft back down until it stabilizes at Cl = 0 (steady state AoA depending on elevon deflection) because Cl = 0 is the steady state where no torque exists. However, it also generates no lift and the aircraft naturally crashes.
This result makes sense to me mathematically, but I struggle to observe this in real aircraft.
In a conventional aircraft the elevator is much farther back than the main wing and can balance the torque at a positive sum of the lift vectors. On a tailless delta aircraft with split elevons, the set of elevons towards the wing root have a CoL different from the elevons near the tips, which would allow small effective Cl according to the logic above. But this seems barely controllable to me and only reliable at high speed to compensate for the abysmal Cl.
Am I making a mistake? Is this really how it works?

Surely pitching upward moves the CoL towards the leading edge, does it not?

Flying wings are normally swept, including the trailing edge, and the elevons are at the tips only. But I am not sure how it works for tailless delta either.

@Frog But not ahead of the CoG, right? So the sign of the torque will not flip.

"CoL"-- it is unclear what you mean by this.  Do you mean the Center of Lift of the wing itself, excluding the effect of the deflected elevons?  Or the Center of Lift of the whole aircraft?  Are you using the Center of Pressure concept where the entire effect of the lift vector is expressed by assuming it acts at the Center of Pressure?  (In contrast to the more modern concept of an Aerodynamic Center that remains stationary, plus a pitching moment coefficient?

I was using it in the Center of Pressure sense. Since the elevons are across the whole wing and this is discussing pitch, the wings are symmetric and elevon deflection just moves the aircraft CoL around.

re last comment-- perhaps the material I've added to the answer will help address this--

Specifically, the simulation suggests to me that a flying wing (or any
tailless aircraft) with two single elevons along the whole of the two
trailing edges is not controllable.

This is certainly not true.  Many radio-controlled model airplanes and gliders with an all-wing, chevron-shaped configuration and no tail have this exact configuration of the elevons, which are the sole control surfaces.

For example, in the photo above, the elevons are red.
Google "Zagi flying wing" for many more examples.

If the elevons are deflected to pitch up, for example, it generates
negative lift over both wings and the torque (assuming CoG is ahead of
CoL) pitches the aircraft up.

This is not true either.  When flying a wing like this, an aft stick input generates a nose-up pitch torque that increases the angle-of-attack of the wing.  In the long run, the wing's lift coefficient is increased, not decreased.  There is no visible evidence of even a temporary tendency for the aircraft to "settle" (accelerate downwards) when the elevons are first deflected upwards, before the wing has had time to rotate to a higher angle-of-attack, though in theory this tendency must exist to some slight degree due to a temporary decrease in the wing's lift coefficient (but not to the extent that it actually becomes negative).  This "settling" tendency might be more significant if the aircraft's pitch rotational inertia were greater.
Conceptually, this is no different than the idea that when we are flying a "conventionally" shaped aircraft, and we move the control stick aft, there must some temporary "settling" tendency (downward acceleration) due to the fact that we've increased the downforce (or in some cases, decreased the upforce) generated by the tail, so the net upward lift generated by the aircraft is now less than weight.  Of course, this situation only persists until the aircraft rotates in the nose-up direction enough to increase the angle-of-attack of the wing sufficiently to bring everything back into balance, at a higher angle-of-attack and lower airspeed than were present initially.  In actual practice, this "settling" tendency is almost always so negligible as to be undetectable.
It might help you understand what is going on if you were to consider reflexed airfoils.  In such an airfoil, the trailing edge of the airfoil is permanently "bent" upwards, contributing a nose-up pitch torque, yet the airfoil still creates positive lift over a wide range of angles-of-attack.
For example:

(Source: https://www.diva-portal.org/smash/get/diva2:450812/FULLTEXT01.pdf )
It appears you are getting into trouble by relying on the idea that the Center of Lift is behind the Center of Gravity.  In reality, the Center of Pressure is not fixed, and in steady-state flight, the Center of Pressure (of the whole aircraft including the contribution from the elevons) and the Center of Gravity must coincide.  When we raise the elevons to create a nose-up pitch torque, this pitch torque can be conceptualized as a forward shift of the Center of Pressure.  Then when the wing rotates to a higher angle-of-attack, the Center of Pressure moves back aft until it coincides with the CG again, and the aircraft ends up in steady-state flight at a higher angle-of-attack and higher lift coefficient and lower airspeed than it started with.
A more modern way to think about aerodynamic forces and torques is to view the aircraft as having an Aerodynamic Center that is fixed at the quarter-chord point of the airfoil, plus a pitching moment coefficient that varies according to the shape of the airfoil.  With this convention, the lift vector can be viewed as acting at the Aerodynamic Center, but we also have to consider the effect of the pitching moment coefficent.  Raising the elevons shifts the pitching moment coefficient in the nose-up direction, as well creating some decrease in the lift coefficient.  A symmetrical airfoil has a pitching moment coefficent of zero, and conventionally shaped cambered airfoil with no reflex has a nose-down pitching moment coefficient.

Right, I was expecting to be wrong because the simulation did not seem realistic to me. But I don't see what precisely the mistake is. Why DOESN'T the aircraft go into a steady state wherever Cl = 0 is?

"This is not true either. When flying a wing like this, an aft stick input generates a nose-up pitch torque that increases the angle-of-attack of the wing. The wing's lift coefficient is increased" This cannot be. An upwards elevon deflection reduces lift on the wing from an airfoil PoV (ignoring AoA for now). Textbooks say that too and you can also see it in XFLR5.

@fnb -- content added-- may help?

Yes, that answers my question! Thank you very much!

What might have been missing to begin with is that the center of gravity is displaced forward of the center of lift to produce stability -- so absent a swept/twisted wing (like some Lippisch designs, or a Klingberg Wing -- or split surfaces, with primarily elevator at the tips and mainly aileron near the roots --  either airfoil reflex or elevon deflection is necessary in steady state to produce longitudinal stability.  Those special cases just divide up which part of the wing is doing what job, of course.

The elevon is creating a localized downward lifting force that subtracts from but doesn't eliminate the overall lift force.  It's just doing the same job as a tail surface, just nestled right up against the wing.  The moment arm is really short, but the balance of forces for trim is the same.

I’m a bit rusty on my aerodynamic theory but having flown the Avro Vulcan B2 for several years I can assure the that a delta wing with elevons works very well!  The only slightly unconventional handling trait was the ‘flap effect’ if you pushed the stick forward on finals and suddenly got a bit of extra lift!


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Is the Reference Speed (Vref) the same as the speed at which the aircraft cross the runway threshold in landing phase?
Just a little confuse when I learned about the aircraft classification based on approach speed. Can anyone help me to have an accuracy look about this question?

Vref is indeed defined as

Landing reference speed or threshold crossing speed.

It is normally set with reference to Vs0 which is stall speed in landing configuration. Vref must usually be greater than 1.3x Vs0.
By way of example, the POH for my PA-28 lists the stall speed with flaps down (ie. landing configuration) as 45KIAS. I tend to be around 70KIAS on final approach and 65KIAS over the threshold. 1.3x my stall speed would be roughly 59KIAS, so I have about 5KIAS safety.

For an airliner, the target for the threshold crossing speed is typically 5 to 15 knots higher than $v_\text{ref}$, depending on headwind.

@Bianfable It surprises me that its not represented still as a factor of vref

Since it's related to wind, I guess they add because the wind speed is additive to the airspeed, but not sure. Airbus has a lengthy article on the topic (search for "Lowest Selectable Speed" and "Approach speed" for the relevant parts).


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What is a Tromboning arrival procedure?
I only know that Tromboning is a RNAV arrival procedure which has the trombone shape, and it's an application of CDO (Continuous Descent Operation) along with Point Merge Procedure. Can you give me some documents related to this procedure, and some example of the airport that have been using this procedure?

Here’s an example of a ‘Trombone’-type arrival into Frankfurt International.

This style of approach works very well for large airports, which have multiple parallel runways for landing e.g. Hartsfield Jackson in Atlanta, LAX, etc.  note Hal, the approach involves downwind legs parallel to the runway that the traffic will be landing on a multiple points along the way, which controllers can sequence large amounts of traffic onto final. This gives a plan view the approach, the appearance of a trombone handle, hence the name. Functionally this kind of instrument approach works similar to a VFR break and landing pattern used by the military for recovering large numbers of aircraft elements and sections quickly, albeit in a much greater volume of space and does not rely on aggressive visual breaks to separate traffic.


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What is the Surface Management (SMAN) all about?
Like AMAN (Arrival Management) and DMAN (Departure Management), what is the SMAN (Surface Management) all about? Is SMAN a studied concepts, or is it a technical system ?
If the SMAN is rather a technical system, so what subsystem does the SMAN included?
I find it hard to figure out how the SMAN can accurately decided the aircraft off-block time ?

Surface Management (SMAN) is a system that assists air traffic / ground traffic controllers to optimise and control traffic flows on the airport's surface. It provides the functionality of the higher service levels of the Advanced Surface Movement Guidance and Constrol System (A-SMGCS) (A-SMGCS Level 3 (Conflict Detection) and
A-SMGCS Level 4 (Conflict Resolution, Automatic Planning & Guidance)) .
Specifically, SMAN provides :

Routing Service: this generates individual routes for aircraft and vehicles based on a start point (i.e. current position) and destination point, taking into account preferred (taxi) routes, taxiway closures, road closure and routing clearances provided to other mobiles. For aircraft, the routing is typically between the stand and the assigned runway entrance holding point, or runway exit point and stand. For vehicles it can be any two positions on the movement area.

Guidance Service: this controls the visual information that flight crews and drivers will receive to help them to follow the assigned route provided by the routing service . This includes control of stop bar lighting and activation of activation of Advanced-Visual Docking Guidance Systems (A-VDGS) at stands. It can also include the switching of the taxiway centre lighting (follow the greens).

Safety Support Service: this provides an alert to controllers when it detects conflicting ATC clearances or when an aircraft/vehicle is not following its cleared route.

References:

DFS SMAN flyerPDF
Skybrary A-SMGCS

So what I get here is SMAN is simply just a Level 3/4 of the A-SMGCS.
And the SMAN does not decided the off-block time, but rather than to help optimize effectively the off-block time, right sir ?
Your two references above is really helpful, if possible, could you give me more references about the SMAN please ?
Really appreciate your answer sir! It's a great help for my research.
Sorry for my poor English if i made any mistake, have a nice day !

The Target Off Block Time is set by the aircraft operator and/or ground handling agent. It is an estimate of when the aircraft is ready to go ( doors closed, boarding bridge removed, push back vehicle available and ready to start up / push back immediately upon reception of clearance from the Tower). This is an input into the SMAN and DMAN. The SMAN can provide accurate estimates of the taxi times to runway entrance holding points, the DMAN keeps a file on (departure) runway usage and slots. Combined, the departure plan is made.